The present invention relates to novel cephalosporin analogs and more specifically to new carbacephem compounds which differ from cephalosporins by having a carbon atom instead of sulphur atom in the dihydrothiazine ring. In the Journal of the American Chemical Society, 96, 7584 (1974) and J. Med. Chem., 20, 551 (1977) certain carbacephems with substituted methyl groups at the C-3 position such as (i)-1-carbacephalotin represented by the formula: ##STR1## are disclosed as having antibacterial activity. Nevertheless as microorganism populations develop immunity to existing antibiotics new antibacterial compounds are in demand. To this end, novel carbacephems having a hydrogen atom at the C-3 position have been synthesized, such compounds and their synthesis being disclosed in commonly owned U.S. patent applications Ser. No. 23,645 and Ser. No. 23,646, filed Mar. 23, 1979. Furthermore, new carbacephem structures with a halogen atom at C-3 position represented by the formula [II] ##STR2## wherein X.sub.1 represents an azido, amino or substituted amino group, Hal represents a halogen atom, and R represents a carboxylic acid or a carboxylic acid ester have been synthesized and these compounds and synthesis thereof are disclosed in commonly owed copending U.S. patent application Ser. No. 107,435, filed Dec. 26, 1979. It has now been found that novel 7-acylamino-3-halogen-substituted-carbacephem compounds have unexpectedly potent antibacterial activity.